The invention relates to a movement detector which is capable of detecting movement of a body, for example a human, in a space and includes a light-sensitive sensor and optical means which are capable of projecting a multiple image of the space onto the sensor.
A movement detector of this kind is known. The optical means in the known detector consists of a plurality of lenses, usually in the form of a multi-faceted lens. The sensor, usually being a passive infrared (PIR) sensor, is arranged, for example, over an opening in a ceiling while the lenses are situated underneath the opening in the ceiling. Each of the lenses projects an image of the space situated therebelow onto the sensor. When a person moves in the space, the (infrared) light intensity projected onto the sensor by the lenses will exhibit fluctuations that can be detected by means of the electronic circuitry coupled to the sensor so that, for example, an alarm can be triggered.
It is a drawback of the known movement detector that the lenses that are fitted underneath the ceiling must have a given cross-section and hence occupy a comparatively large surface area of a diameter of a few centimeters, so that the movement detector can be easily discovered by an unwanted person, such as a burglar. Moreover, such a comparatively large detector is experienced as a displeasing element on the ceiling.
The more accurate the detector must be, the more images of the space must in principle be projected onto the sensor, and hence the more lenses are required. Furthermore, the assembly of lenses should be constructed in such a manner that each individual lens is focused onto the sensor element for the detection distance applicable to this individual lens. This makes it practically impossible to vary the distance between the lens assembly and the sensor.
It is an object of the invention to provide a simple, inexpensive, sensitive and reliable motion detector that can be used in a variety of ways and can be installed in a space in such a manner that it is less conspicuous.
To this end, the optical means include a mirror assembly having a kaleidoscopic effect. Because of the kaleidoscopic effect, the space is imaged onto the sensor in multiple form and, when the mirror assembly forms a closed circumference, in principle in an infinite multiple, so that a very accurate sensor can be realized. The movement detector can be arranged in the ceiling in such a manner that only the mirror assembly projects from the ceiling. The cross-section of this mirror assembly need only amount to a few millimeters, so that the detector can hardly be noticed. The mirror assembly preferably constitutes an elongate body whose reflecting surface faces inwards. This body may be hollow and be formed by mirrors; it may also be formed by a solid body that is transparent to the relevant light, for example a glass body whose side faces constitute inwards facing mirrors, either by interface reflection or by way of an externally deposited mirror layer. An assembly of mirrors having a kaleidoscopic effect is known per se and described, for example in the patent documents GB-A-2 228 098 and JP-A-7 236 775.
In order to enhance appropriate operation of the movement detector it is necessary to image the space actually on the sensor. To this end, the optical means preferably also include a lens. The sensor is then preferably situated near a first end of the mirror assembly whereas the lens is preferably situated near the second, opposite end of the mirror assembly.
It is known per se that a kaleidoscopic effect can be obtained by making the cross-section of an assembly of mirrors form a polygon. In the simplest but efficient implementation the cross-section of the mirror assembly is shaped essentially as a triangle, that is, preferably an isosceles triangle.
The cross-section of the mirror assembly in a simple but effective embodiment is essentially the same along its entire longitudinal axis. However, if a larger or a smaller part of the space is to be imaged onto the sensor, it may be advantageous to make the cross-section of the mirror assembly vary along its longitudinal axis as from the sensor, that is, from a smallest to a largest cross-section or from a largest to a smallest cross-section. A xe2x80x9cwide anglexe2x80x9d effect and a xe2x80x9ctele-effectxe2x80x9d, respectively, are thus achieved.
The sensor preferably includes an infrared sensor which is, for example, sensitive to light of a wavelength in the range of from 3 to 10 xcexcm.
The invention also relates to a method of installing a movement detector in a space in order to detect movement of a body in the space, a light-sensitive sensor being arranged above a ceiling of the space while optical means which include a mirror assembly having a kaleidoscopic effect are arranged in such a manner that they project a multiple image of the space onto the sensor, the arrangement being such that the mirror assembly extends essentially through the ceiling.